* Size difference: Koi fish are significantly larger than bettas. Even the smallest koi are much bigger than the largest betta. A betta simply wouldn't be able to swallow a koi.
* Aggression levels: While bettas can be aggressive towards other bettas, they are generally not aggressive towards much larger fish.
* Diet: Bettas are carnivorous and eat smaller insects and crustaceans. Koi are herbivores and eat plants and algae.
It's important to remember that keeping fish of vastly different sizes and aggression levels together can be dangerous and stressful for both fish. Always research the specific needs of your fish before putting them in the same tank.